1. Field of Invention
The invention pertains generally to inertial sensors and the like and, more particularly to a micromachined vibratory gyroscope.
2. Related Art
Vibratory gyroscopes operate by detecting Coriolis-induced motion induced by rotation of the gyroscope about a sensitive axis. When a mass is driven to oscillate along an given axis and is rotated about an axis perpendicular to the axis of vibration, a Coriolis force is generated and applied to the mass along a response axis perpendicular to the axes of vibration and rotation. The rate of rotation is measured by detecting the change in motion of the mass along the response axis caused by the Coriolis force.
Coriolis-induced forces on the vibrating masses are in phase with the velocity of the masses since the Coriolis force is proportional to the velocity. Any undesired coupling of the motion along the primary or driven axis of vibration to the response axis will give rise to a spurious motion of the masses along the response axis. This undesired coupling is generally in phase with the displacement of the masses, rather than velocity, and is often referred to as a quadrature error.
One way to sense a change in motion of a mass due to a Coriolis force is capacitive detection, which typically involves a fixed electrode and a movable electrode. In such devices, it is important to minimize motion of the movable electrode in the absence of applied rotation, i.e., any motion of the mass along the response axis which is not due to a Coriolis force. Otherwise, an undesired quadrature signal will be present, having the same frequency as the rate signal but phase shifted by 90 degrees. This quadrature signal is superimposed on the desired output signal. Although the quadrature signal can be partially rejected electronically, e.g. by the use of phase-sensitive demodulation, that tends to degrade the performance of the gyroscope.
Another source of error in a vibratory gyroscope is sensitivity to linear accelerations which displace the masses thus produce undesired outputs.
When a gyroscope is mounted on a support for a given application, any unbalanced momentum of the vibrating masses will cause part of the driving energy to be injected into the support and then potentially be coupled back to the device. Energy fed back in that manner can cause bias errors and makes the performance of the device sensitive to the mounting conditions.
In micromachined vibratory gyroscopes of the prior art, the vibrating masses are generally coupled together by mechanical means. The coupling is important in order to assure that the masses will oscillate at the same frequency of resonance. Uncoupled masses would tend to have different resonant frequencies, which would not be conducive to a practical sensor.
While mechanical coupling does assure the masses will vibrate with a single frequency of resonance, such couplings also have certain limitations and disadvantages. For example, they are prone to variations in dimension due to fabrication tolerances, causing the degree of coupling to be variable. Also, many of them employ folded beam designs which increase the required substrate area and size of the device. Moreover, the degree of coupling is determined by the fixed mechanical properties of the coupling structure, and is not adjustable.